One of the most widely used deposition methods in semiconductor fabrication is chemical vapor deposition. In chemical vapor deposition (CVD), gaseous molecules in the form of reactant gases are introduced into a reactor and undergo chemical reactions for conversion into the desired deposit material. For example, SiH.sub.4 molecules decompose into silicon atoms (the desired deposit material) and H.sub.2 molecules. The silicon atoms are adsorbed on the surface of a substrate and the H.sub.2 molecules desorb or leave the surface. The adsorbed silicon atoms eventually become chemically bonded to the substrate and form a layer of silicon on the surface.
Early CVD methods used a relatively high temperature and thermally-activated chemistry to deposit material from a reactant gas onto a heated substrate positioned inside a reactor. It was found that some of the CVD processes could be "enhanced" by adding plasma to the reactant gas. The term plasma, as used in CVD processes, means a partially ionized gas.
In semiconductor processes, only a small fraction of the reactant gas needs to be ionized. However, the plasma reactor contains a large percentage of species called radicals, which are highly reactive excited gas species. These radicals, which do not normally exist without a plasma, are created by gas phase collisions with electrons and/or ionized species driven by an electric field. As a result, some CVD processes can use plasma enhancement to increase the reaction rate of a chemical reaction, or drive a chemical reaction that does not occur at all without the plasma. For example, many processes do not occur if the temperature is too low, unless assisted by the presence of a plasma.
As a result of plasma-enhanced deposition techniques, low temperature CVD processes have been developed for forming diverse materials, including metals such as aluminum and tungsten, dielectric films such as silicon nitride and silicon dioxide, and semiconductor films such as silicon. The low temperature method is desirable because higher temperature may damage substrate structure. However, one of the problems of prior art plasma-enhanced deposition systems is that deposition near the rim of a substrate is not uniform.
Plasma is generated by applying an electric field of sufficient intensity to a gas. A common configuration is to apply a high voltage power source to a pair of parallel electrodes to generate the necessary electric field. When appropriate conditions are reached (e.g., the voltage across the electrodes and the pressure of the gas are at a certain value), some of the gas molecules become ionized and a plasma is formed.
In semiconductor processing, a metal susceptor (for holding a substrate and transmitting heat thereto) is usually used as one of the electrodes. This arrangement simplifies the design of a reactor because the same component can be used for multiple purposes. It makes full use of the properties of metal, i.e., is a good thermal conductor (the metal susceptor can conduct heat from a heat source to the substrate efficiently) and is a good electrical conductor (it can function as an electrode). In this arrangement, an upper electrode is typically connected to a radio frequency source and the metal susceptor is grounded. The difference in potential between the upper electrode and the grounded susceptor creates the electric field which generates a plasma.
It is desirable to have a uniform electric field oriented essentially perpendicular to the metal susceptor, and consequently the substrate. The uniform electric field allows plasma and radicals to be formed uniformly across the span of the surface of the substrate. As a result, the deposition of material on the surface of the substrate is also uniform.
The metal susceptor also functions as a holder for supporting the substrate and normally has a lip around the rim for preventing the substrate from moving outside of the susceptor. However, the lip causes the electric field to deviate from a uniform and perpendicular geometry at the region near the lip. As a result, the layer of deposited material near the rim of the substrate is usually not uniform. Consequently, there is a need for a system which is able to render the deposition uniform even at the rim of the substrate.